1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to an improved attaching structure for removably mounting the top nozzle as a unitary subassembly on the guide thimbles of a reconstitutable fuel assembly.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length. The grids are attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles which extend above and below the ends of the fuel rods. At the top end of the assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in the Klumb et al patents (U.S. Pat. Nos. 3,770,583 or 3,814,667).
During operation of such assembly in a nuclear reactor, the fuel rods may occasionally develop cracks along their length resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of an integral assembly of guide thimbles welded to the top and bottom nozzles, it is difficult to detect and remove the failed rods. To gain access to these rods, it is necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the control rod guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in a reactor because of the damage done to both the guide thimbles and the nozzles which prohibit rewelding.
In view of the high costs associated with replacing fuel assemblies, both domestic and foreign utilities have indicated an interest in reconstitutable fuel assemblies in order to minimize their operating and maintenance expenses.
Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal of individual failed fuel rods, the option to replace rods, followed by the additional use in the reactor and/or normal handling and storage of the affected fuel assembly. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble assembly, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in neutron-absorbing liquid. With rod removal/replacement and after the top nozzle has been remounted on the control rod guide thimbles, the reconstituted assembly can then be reinserted into the reactor and used until the end of its useful life, and/or stored in spent fuel pools or other places in a safe, normal manner.
One type of such reconstitutable fuel assembly can be seen in the aforementioned Klumb et al patents. The fuel assembly of Klumb et al includes a top nozzle which incorporates a hold-down plate and also coil springs coaxially disposed about upwardly extending alignment posts. The alignment posts extend through an upper end plate, spaced below the hold-down plate, and are joined thereto and to the upper ends of the guide thimbles with fastener nuts located on the underside of the upper plate. The upper hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the end plate. A radially enlarged shoulder on the upper end of each of the alignment posts retains the hold-down plate on the posts. In an attempt to improve upon the Klumb et al device, Anthony et al set forth another threaded joint arrangement as seen in U.S. Pat. No. 3,992,259. Yet another type of threaded arrangement used for removably attaching the top nozzle on the control rod guide thimbles can be seen in U.S. Pat. No. 3,828,868 to Jabsen.
These prior art reconstitutable fuel assemblies involve top nozzle arrangements which are difficult to remove and reattach both due to the locations of the fasteners and because removal appears to cause the hold-down device of the nozzle to come apart, requiring added steps and apparatus to prevent this or to later reassemble the hold-down device. Therefore, what has been lacking and is urgently needed is a reconstitutable fuel assembly employing a simple joining or coupling arrangement which allows for easy, remote removal and reattachment of the top nozzle without the possibility of the hold-down device coming apart.